Stress on corn at pollination

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There is not a lot of good news for the 2012 Illinois corn crop, but there has been recent rainfall in some places, and 46% of the crop was silking by July 1, far earlier than normal. Silks are abundant, and leaf color remains good in fields where plants have had enough water to get to a height of 5 ft. or more.

“The early start to pollination is a very positive development, and the fact that even in dry areas the root system has managed to find enough water to get to this point bodes well,” said University of Illinois crop sciences professor Emerson Nafziger.

Unfortunately, the serious problem of water stress and loss of yield potential continues to gain importance. “As we continue without rain over most of the state, we expect this only to worsen,” he predicted.

The worst news is that there’s little hope for some fields, especially in southeastern Illinois. “The plants in some fields are dead. If they are alive but past the pollination stage with very short plants and no kernels, or they have lost most or all of their green color, there’s no chance that they can come back,” Nafziger explained.

Only 26% of the crop was rated good-excellent on July 1. Barely 10% of the acreage has adequate topsoil moisture. Temperatures remain high, and warm nights continue to push GDD accumulations above normal, although the statewide GDD accumulation since May 1 is only 135 above average.

The fact that lack of water for growth is causing tassels to struggle to emerge may have negative consequences for pollination. Pollen is shed as relative humidity drops, so exposed tassel branches usually start to shed 2 to 3 hrs. after sunrise. If the tassel is wrapped inside two leaves, relative humidity stays high longer, delaying pollen shed by as much as several hours.

“In some fields, more pollen is being shed in early afternoon than in the normal mid-morning period,” said Nafziger. “The problem comes when the temperature is above 90 degrees when the most pollen was being shed. At such temperatures, silks are often not as receptive as they would have been at 70 to 75 degrees earlier in the day.”

“We also expect that silk numbers may lag due to water shortages,” he continued. “If plants are struggling to take up enough water to push tassels out above the leaves, we can expect silks to struggle as well, at least those that emerge late.”

Even in fields showing silks and tassels, fertilizing kernels and keeping them going until grainfill begins may not be fully successful in the drier fields. Moreover, as large areas of the state suffer from lack of rain, the number of kernels fertilized may end up being considerably larger than the number that survive to fill and produce yield.

“We remain optimistic that kernel numbers will be okay in many fields, but in the past two years we have had a great deal of kernel abortion, and there is every reason to believe that this will recur in 2012,” Nafziger said.

The factors that affect whether fertilized kernels abort are not entirely understood, but the supply of sugars available to the developing ear is known to be important for keeping productive kernels. Sugars are produced by photosynthesis. Plants that are rolling their leaves for most of the afternoon, as is happening in east central Illinois, are producing less sugar than normal.

“It’s highly likely that there will be fewer than normal numbers of kernels by the time kernel filling starts several weeks from now,” Nafziger predicted.

As stress conditions continue, a reduction in kernel size often starts to develop somewhere along the length of the cob, indicating the start of kernel loss from abortion, which is probably irreversible. The decreased number of kernels that fill is likely to be a primary yield barrier in fields under stress during pollination.

If conditions improve during grainfill, the reduced number of kernels may be able to get a little larger than they normally would. A healthy canopy increases the plants’ potential to make larger kernels. “However, we would not expect an increase in kernel size to make up much of the yield loss due to reduced kernel numbers,” Nafziger noted.

In areas that have been under stress for weeks, one of the first signs of rapid deterioration in yield potential is, or will be, loss of canopy color. Plants that are struggling to take up water are also not taking up much nitrogen, and leaves become less flexible with age. Thus, loss of canopy color after pollination often cannot be reversed completely, even after it rains.

Besides canopy color, light interception serves as an indicator of stress and photosynthetic capacity. As the leaves roll, light interception drops quickly. One of the best “drive-by” indicators of how a crop is doing at a given time is the amount of light that is hitting the ground. In fields where well over half the sunlight is reaching the ground, little photosynthesis is going on.

Based on this indicator, corn following corn seems to be struggling more than corn following soybean, perhaps due in part to incomplete recharge in some areas after last season. Fields that were in first-year corn in 2011 and second-year corn in 2012 may suffer more because both yields and water extraction in 2011 may have been higher.

Daily wilting is taking a toll, primarily by diminishing the energy (sugar) that keeps leaf tissues healthy and repairs damage, but also by causing damage itself. The loss of water vapor through the stomata normally cools the leaf. Leaf temperatures rise above air temperature when the leaves are not getting enough water to keep stomata open. Moreover, sunlight energy that cannot be used in photosynthesis when stomata are closed can damage chlorophyll and the photosynthetic apparatus, leading eventually to loss of leaf color.

Many producers are trying to guess how much corn yield potential has already been lost. In the majority of fields — those that are pollinating now or that will be pollinating in the next week under conditions of at least moderate stress — the first yield potential estimates will have to wait until it is possible to count kernel numbers and get some idea of grainfilling conditions at stage R3 (roasting ear) during the last third of July.

“I wish we could be more optimistic,” said Nafziger. “The start to the growing season was outstanding, and most producers ‘did everything right’ to establish good yield potential. There’s not much we can do other than accept that the weather is beyond our control.”